An automatic recount has been triggered in the U.S. Senate race in Pennsylvania between Bob Casey and Dave McCormick. Here's how the recount will work.

HARRISBURG, Pa. (AP) — The U.S. Senate election in Pennsylvania between Democratic incumbent Sen. Bob Casey and Republican David McCormick is headed for a statewide recount, as counties continued Wednesday to sort through outstanding ballots and the campaigns jousted over which ones should count.

The Pennsylvania Senate race between incumbent Bob Casey, left, and his Republican challenger, Dave McCormick, is headed for a recount. All counties must complete their recounts by Tuesday, Nov. 26. Pennsylvania's Senate race is going to a mandatory recount despite the Associated Press calling it last week for Republican challenger Dave McCormick.

Pennsylvania’s nationally watched U.S. Senate race is headed to a recount after incumbent Sen. Bob Casey did not waive an automatic recount. The Associated Press called the race last week for Republican Dave McCormick,

Pennsylvania Secretary of the Commonwealth Al Schmidt announced ... and he has spent this week in Washington going through the Senate orientation process. "The people have spoken. There's a clear, clear victory," McCormick said last week.

Republican Dave McCormick leads Democratic Sen. Bob Casey by about 28,000 votes, under the 0.5 percentage point threshold for an automatic recount under state law.